Personally, while the buyer might be a total dick, I don't think the seller too can come out of this smelling of roses.
OP, while your ad does not specifically say your harddrive does not come from Apple, do take note that Apple does have 500GB hard drive offerings listed in their store too. you have to explicitly state that yours does not come with the stock option or it will be implied and assumed that you are offering genuine Apple provided parts. It may have been a fallacy of the buyer to not check with you, nevertheless, there may be recourse from the buyer because you offered a product similar to what Apple offers and unintentionally sold it off as genuine.
My take on this. Its your mistake, so just send the stock drive to him, because if the buyer is aggressive, regardless of chargeback, or Paypal, etc... there will be a chance he will get his money back. It makes good sense to rectify your own problem, even if it means a loss to you. Lying may not be a good thing to do on the art of the buyer, but he had every reason to seek reprieve from you because you made the mistake.